This application aims to investigate bone marrow (BM)-derived precursor cells in the context of severe, angioproliferative pulmonary arterial hypertension (PAH, a group of nearly always lethal diseases. The incidence of pulmonary hypertension (PH) appears to be increasing, which might also be related to more frequent diagnosis due to more common use of echocardiography. PH has also been recognized as an indicator of increased mortality in patients with interstitial lung diseases. The 4th WHO conference in Dana Point updated the pathobiological concepts of severe PH, yet no curative therapy has been found so far. It is increasingly acknowledged that the cells in the angioproliferative lesions are apoptosis-resistant, and the recent years have generated substantial information about the contribution of BM-derived precursor cells to vascular remodeling, but it has not been investigated whether or not BM-derived precursor cells participate in the development of angioproliferative lesions in PAH. Our group has established the SU5416/chronic hypoxia (SuHx) model as a model of severe angioproliferative PAH. Our central hypothesis is that BM-derived precursor cells are important and contribute to the development of angioproliferative lesions via growth factor production and engraftment into the vessel wall. Our preliminary data indicate the presence of pluripotent cells in and around small pulmonary arteries in SuHx animals. In the course of the proposed experiments, we will establish various techniques, including BM reconstitution and FACS analysis as tools to elucidate the role of bone-marrow derived precursor cells in the development of angioproliferative lesions in severe experimental PAH. The hypothesis that BM-derived precursor cells contribute significantly to angioproliferation in PAH will be tested in two Specific Aims: In Specific Aim 1, we will answer the question whether BM-derived precursor cells are increased in angioproliferative lesions and blood, and show enhanced production of growth factors, in severe, angioproliferative PAH. In Specific Aim 2, we will investigate the mechanisms of BM-derived precursor cell recruitment into the pulmonary vasculature in severe angioproliferative PAH. We will specifically examine whether apoptosis inhibition and blocking CXCR4 will decrease precursor cell homing, angioproliferation and PH. In sum, this project aims to demonstrate that BM-derived precursor cells contribute to angioproliferation in PAH via growth factor production and engraftment into the vessel wall.